The AR-15 is a lightweight, 5.56 mm, air-cooled, gas-operated, magazine-fed semi-automatic rifle, with a rotating-lock bolt. The main mechanism of operation for the rifle is known as direct gas impingement. Gas is trapped from the barrel as the bullet moves past a gas port located above the rifle's front sight base. The gas rushes into the port and down a gas tube, located above the barrel, which runs from the front sight base into the AR-15's upper receiver. Here, the gas tube protrudes into a “gas key” (bolt carrier key) which accepts the gas and funnels it into the bolt carrier.
The bolt and bolt carrier together effectively form a piston, which is caused to move as the cavity in the bolt carrier fills with high pressure gas. The bolt is locked into the barrel extension, so this expansion forces the bolt carrier backward a short distance in line with the stock of the rifle to first unlock the bolt. As the bolt carrier moves toward the butt of the gun, the bolt cam pin, riding in a slot on the bolt carrier, forces the bolt to turn and unlock from the barrel extension. Once the bolt is fully unlocked it begins its rearward movement along with the bolt carrier. The bolt's rearward motion extracts the empty cartridge case from the chamber, and as soon as the neck of the case clears the barrel extension, the bolt's spring-loaded ejector forces it out the ejection port in the side of the upper receiver. The bolt is much heavier than the projectile, and along with the recoil-spring pressure inside the stock buffer-tube performs the cartridge ejection function and chambers the following cartridge.
Behind the bolt carrier is a plastic or metal buffer which rests in line with a bolt return spring that pushes the bolt carrier back toward the chamber to return the bolt into battery. A groove machined into the upper receiver traps the cam pin and prevents it and the bolt from rotating into a closed position. The bolt's locking lugs then push a fresh round from the magazine which is guided by feed ramps into the chamber. As the bolt's locking lugs move past the barrel extension, the cam pin is allowed to twist into a pocket milled into the upper receiver. This twisting action follows the groove cut into the carrier and forces the bolt to twist and “lock” into the barrel's unique extension.
The bolt return spring is a simple coil spring that is confined by the outer walls of the extension tube. The spring rubs against the walls as it is compressed and extended. In some rifles, the result is a loud and annoying buzzing sound that can exceed a second in duration after each round is fired.
One attempt to address this problem is the replacement of the standard recoil spring buffer with a pneumatic piston system that uses compressed gas to store and return the recoil energy. However, this approach works only with rifles having buttstocks with full length extension tubes. Rifles or carbines having collapsible stocks, which have shorter extension tubes, cannot accommodate the pneumatic system. Pneumatic systems are also believed to have limited reliability and utility in extreme temperature environments.
Therefore, a need exists for a new and improved captured spring assembly for a firearm that reduces the noise of the bolt return spring of a firearm with either a full length or collapsible stock. In this regard, the various embodiments of the present invention substantially fulfill at least some of these needs. In this respect, the captured spring assembly for a firearm according to the present invention substantially departs from the conventional concepts and designs of the prior art, and in doing so provides an apparatus primarily developed for the purpose of constraining movement of the bolt return spring of a firearm by capturing the spring on an inner rod.